Google researchers have warned of a huge rise in fake anti-virus software that tricks users into paying for a useless software suite.

Fake anti-virus software now accounts for 15 per cent of all online malware, and is responsible for 50 per cent of all malware delivered by advertising, a fivefold increase in a single year.

“Fake anti-virus attacks spread easily without requiring any vulnerability on a victim’s computer system,” said the team in a paper entitled The Nocebo Effect on the Web (PDF).

“Additionally, fake anti-virus distributors attempt to maximise their reach by posting ads that lead to the fake anti-virus distribution sites, or funnelling traffic through search engine optimised web sites designed to rank highly for popular keywords.”

Using data from 2009 the team saw a rise in fake anti-virus software from three per cent of infected domains to 15 per cent. In January 2009 there were 93 domains hosting fake anti-virus software but by the end of the year this had risen to 587.

The team also reported that legitimate security software is having increasing problems identifying the fake anti-virus code, and that Google has developed software to speed up detection.

Fake anti-virus adverts have been growing steadily, as they can be highly profitable and easy to set up. In addition they can function on fully patched PCs since they exploit no software vulnerabilities.